dpkg-deb verifies that conffiles exist, so verify that the specified files
actually exist before writing them to conffiles.
This mirrors the behaviour of FILES and package_rpm's CONFFILES handling.
(From OE-Core rev: 767a3d1c45f97add720174a7034df750bfb09a8e)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>